The Guardian (or at least the PA) reports that Prince Charles' income from the Duchy of Cornwall exceeded NZ$30mln last year. In doing so, it repeats the common assertion that the Duchy is the private property of the Prince of Wales and that he receives no income from the state.
I did some analysis of this - the Duchy was founded in 1337 by the Black Prince. In those days, the monarchy equalled the nation state - personal and national possessions were not differentiated. Since then Britain has become a constitutional monarchy, and there is a clear distinction between national property (e.g. Hyde Park) and personal possessions (e.g. Prince Harry's paycheque from the Army).
Which does the Duchy fall into?
When founded, it was part of the combined national/royal assets. In 1760, most of the royal assets were accepted as national property (and the state agreed to provide monarchs with an income, the Civil List). The Duchy of Cornwall was not included in this - I don't have any background on why? It was certainly not (like Harry's salary or his inheritance from Diana) the result of personal, as opposed to public, endeavors.
The Duchy is also not like a normal asset - it is not passed automatically to the holders heirs, and it can be held in abeyance for several years. (most recently from 1936-1952). Being Duke goes with being the oldest son of the monarch - if no such person exists, then there is no dukedom and the income from the Duchy goes to the state.
Hence, I would conclude that the money from the Duchy, rather than being private wealth, represents an income from the nation for being the monarchs eldest son. This being so, one must ask whether it represents a good deal for the UK taxpayer?